java.util.concurrent.locks
Class AbstractQueuedSynchronizer

Provides a framework for implementing blocking locks and related
synchronizers (semaphores, events, etc) that rely on
first-in-first-out (FIFO) wait queues. This class is designed to
be a useful basis for most kinds of synchronizers that rely on a
single atomic int value to represent state. Subclasses
must define the protected methods that change this state, and which
define what that state means in terms of this object being acquired
or released. Given these, the other methods in this class carry
out all queuing and blocking mechanics. Subclasses can maintain
other state fields, but only the atomically updated int
value manipulated using methods getState(), setState(int) and compareAndSetState(int, int) is tracked with respect
to synchronization.

Subclasses should be defined as non-public internal helper
classes that are used to implement the synchronization properties
of their enclosing class. Class
AbstractQueuedSynchronizer does not implement any
synchronization interface. Instead it defines methods such as
acquireInterruptibly(int) that can be invoked as
appropriate by concrete locks and related synchronizers to
implement their public methods.

This class supports either or both a default exclusive
mode and a shared mode. When acquired in exclusive mode,
attempted acquires by other threads cannot succeed. Shared mode
acquires by multiple threads may (but need not) succeed. This class
does not "understand" these differences except in the
mechanical sense that when a shared mode acquire succeeds, the next
waiting thread (if one exists) must also determine whether it can
acquire as well. Threads waiting in the different modes share the
same FIFO queue. Usually, implementation subclasses support only
one of these modes, but both can come into play for example in a
ReadWriteLock. Subclasses that support only exclusive or
only shared modes need not define the methods supporting the unused mode.

This class defines a nested AbstractQueuedSynchronizer.ConditionObject class that
can be used as a Condition implementation by subclasses
supporting exclusive mode for which method isHeldExclusively() reports whether synchronization is exclusively
held with respect to the current thread, method release(int)
invoked with the current getState() value fully releases
this object, and acquire(int), given this saved state value,
eventually restores this object to its previous acquired state. No
AbstractQueuedSynchronizer method otherwise creates such a
condition, so if this constraint cannot be met, do not use it. The
behavior of AbstractQueuedSynchronizer.ConditionObject depends of course on the
semantics of its synchronizer implementation.

This class provides inspection, instrumentation, and monitoring
methods for the internal queue, as well as similar methods for
condition objects. These can be exported as desired into classes
using an AbstractQueuedSynchronizer for their
synchronization mechanics.

Serialization of this class stores only the underlying atomic
integer maintaining state, so deserialized objects have empty
thread queues. Typical subclasses requiring serializability will
define a readObject method that restores this to a known
initial state upon deserialization.

Each of these methods by default throws UnsupportedOperationException. Implementations of these methods
must be internally thread-safe, and should in general be short and
not block. Defining these methods is the only supported
means of using this class. All other methods are declared
final because they cannot be independently varied.

Even though this class is based on an internal FIFO queue, it
does not automatically enforce FIFO acquisition policies. The core
of exclusive synchronization takes the form:

Acquire:
while (!tryAcquire(arg)) {
enqueue thread if it is not already queued;
possibly block current thread;
}
Release:
if (tryRelease(arg))
unblock the first queued thread;

(Shared mode is similar but may involve cascading signals.)

Because checks in acquire are invoked before enqueuing, a newly
acquiring thread may barge ahead of others that are
blocked and queued. However, you can, if desired, define
tryAcquire and/or tryAcquireShared to disable
barging by internally invoking one or more of the inspection
methods. In particular, a strict FIFO lock can define
tryAcquire to immediately return false if getFirstQueuedThread() does not return the current thread. A
normally preferable non-strict fair version can immediately return
false only if hasQueuedThreads() returns
true and getFirstQueuedThread is not the current
thread; or equivalently, that getFirstQueuedThread is both
non-null and not the current thread. Further variations are
possible.

Throughput and scalability are generally highest for the
default barging (also known as greedy,
renouncement, and convoy-avoidance) strategy.
While this is not guaranteed to be fair or starvation-free, earlier
queued threads are allowed to recontend before later queued
threads, and each recontention has an unbiased chance to succeed
against incoming threads. Also, while acquires do not
"spin" in the usual sense, they may perform multiple
invocations of tryAcquire interspersed with other
computations before blocking. This gives most of the benefits of
spins when exclusive synchronization is only briefly held, without
most of the liabilities when it isn't. If so desired, you can
augment this by preceding calls to acquire methods with
"fast-path" checks, possibly prechecking hasContended()
and/or hasQueuedThreads() to only do so if the synchronizer
is likely not to be contended.

This class provides an efficient and scalable basis for
synchronization in part by specializing its range of use to
synchronizers that can rely on int state, acquire, and
release parameters, and an internal FIFO wait queue. When this does
not suffice, you can build synchronizers from a lower level using
atomic classes, your own custom
Queue classes, and LockSupport blocking
support.

Usage Examples

Here is a non-reentrant mutual exclusion lock class that uses
the value zero to represent the unlocked state, and one to
represent the locked state. It also supports conditions and exposes
one of the instrumentation methods:

AbstractQueuedSynchronizer

Creates a new AbstractQueuedSynchronizer instance
with initial synchronization state of zero.

Method Detail

getState

protected final int getState()

Returns the current value of synchronization state.
This operation has memory semantics of a volatile read.

Returns:

current state value

setState

protected final void setState(int newState)

Sets the value of synchronization state.
This operation has memory semantics of a volatile write.

Parameters:

newState - the new state value

compareAndSetState

protected final boolean compareAndSetState(int expect,
int update)

Atomically sets synchronization state to the given updated
value if the current state value equals the expected value.
This operation has memory semantics of a volatile read
and write.

Parameters:

expect - the expected value

update - the new value

Returns:

true if successful. False return indicates that
the actual value was not equal to the expected value.

tryAcquire

protected boolean tryAcquire(int arg)

Attempts to acquire in exclusive mode. This method should query
if the state of the object permits it to be acquired in the
exclusive mode, and if so to acquire it.

This method is always invoked by the thread performing
acquire. If this method reports failure, the acquire method
may queue the thread, if it is not already queued, until it is
signalled by a release from some other thread. This can be used
to implement method Lock.tryLock().

arg - the acquire argument. This value
is always the one passed to an acquire method,
or is the value saved on entry to a condition wait.
The value is otherwise uninterpreted and can represent anything
you like.

Returns:

true if successful. Upon success, this object has been
acquired.

Throws:

IllegalMonitorStateException - if acquiring would place
this synchronizer in an illegal state. This exception must be
thrown in a consistent fashion for synchronization to work
correctly.

arg - the release argument. This value
is always the one passed to a release method,
or the current state value upon entry to a condition wait.
The value is otherwise uninterpreted and can represent anything
you like.

Returns:

true if this object is now in a fully released state,
so that any waiting threads may attempt to acquire; and false
otherwise.

Throws:

IllegalMonitorStateException - if releasing would place
this synchronizer in an illegal state. This exception must be
thrown in a consistent fashion for synchronization to work
correctly.

tryAcquireShared

protected int tryAcquireShared(int arg)

Attempts to acquire in shared mode. This method should query if
the state of the object permits it to be acquired in the shared
mode, and if so to acquire it.

This method is always invoked by the thread performing
acquire. If this method reports failure, the acquire method
may queue the thread, if it is not already queued, until it is
signalled by a release from some other thread.

arg - the acquire argument. This value
is always the one passed to an acquire method,
or is the value saved on entry to a condition wait.
The value is otherwise uninterpreted and can represent anything
you like.

Returns:

a negative value on failure, zero on exclusive success,
and a positive value if non-exclusively successful, in which
case a subsequent waiting thread must check
availability. (Support for three different return values
enables this method to be used in contexts where acquires only
sometimes act exclusively.) Upon success, this object has been
acquired.

Throws:

IllegalMonitorStateException - if acquiring would place
this synchronizer in an illegal state. This exception must be
thrown in a consistent fashion for synchronization to work
correctly.

arg - the release argument. This value
is always the one passed to a release method,
or the current state value upon entry to a condition wait.
The value is otherwise uninterpreted and can represent anything
you like.

Returns:

true if this object is now in a fully released state,
so that any waiting threads may attempt to acquire; and false
otherwise.

Throws:

IllegalMonitorStateException - if releasing would place
this synchronizer in an illegal state. This exception must be
thrown in a consistent fashion for synchronization to work
correctly.

acquire

public final void acquire(int arg)

Acquires in exclusive mode, ignoring interrupts. Implemented
by invoking at least once tryAcquire(int),
returning on success. Otherwise the thread is queued, possibly
repeatedly blocking and unblocking, invoking tryAcquire(int) until success. This method can be used
to implement method Lock.lock()

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquire(int) but is
otherwise uninterpreted and can represent anything
you like.

acquireInterruptibly

Acquires in exclusive mode, aborting if interrupted.
Implemented by first checking interrupt status, then invoking
at least once tryAcquire(int), returning on
success. Otherwise the thread is queued, possibly repeatedly
blocking and unblocking, invoking tryAcquire(int)
until success or the thread is interrupted. This method can be
used to implement method Lock.lockInterruptibly()

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquire(int) but is
otherwise uninterpreted and can represent anything
you like.

tryAcquireNanos

Attempts to acquire in exclusive mode, aborting if interrupted,
and failing if the given timeout elapses. Implemented by first
checking interrupt status, then invoking at least once tryAcquire(int), returning on success. Otherwise, the thread is
queued, possibly repeatedly blocking and unblocking, invoking
tryAcquire(int) until success or the thread is interrupted
or the timeout elapses. This method can be used to implement
method Lock.tryLock(long, TimeUnit).

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquire(int) but is
otherwise uninterpreted and can represent anything
you like.

acquireShared

public final void acquireShared(int arg)

Acquires in shared mode, ignoring interrupts. Implemented by
first invoking at least once tryAcquireShared(int),
returning on success. Otherwise the thread is queued, possibly
repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success.

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquireShared(int) but is
otherwise uninterpreted and can represent anything
you like.

acquireSharedInterruptibly

Acquires in shared mode, aborting if interrupted. Implemented
by first checking interrupt status, then invoking at least once
tryAcquireShared(int), returning on success. Otherwise the
thread is queued, possibly repeatedly blocking and unblocking,
invoking tryAcquireShared(int) until success or the thread
is interrupted.

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquireShared(int) but is
otherwise uninterpreted and can represent anything
you like.

tryAcquireSharedNanos

Attempts to acquire in shared mode, aborting if interrupted, and
failing if the given timeout elapses. Implemented by first
checking interrupt status, then invoking at least once tryAcquireShared(int), returning on success. Otherwise, the
thread is queued, possibly repeatedly blocking and unblocking,
invoking tryAcquireShared(int) until success or the thread
is interrupted or the timeout elapses.

Parameters:

arg - the acquire argument.
This value is conveyed to tryAcquireShared(int) but is
otherwise uninterpreted and can represent anything
you like.

hasQueuedThreads

public final boolean hasQueuedThreads()

Queries whether any threads are waiting to acquire. Note that
because cancellations due to interrupts and timeouts may occur
at any time, a true return does not guarantee that any
other thread will ever acquire.

In this implementation, this operation returns in
constant time.

Returns:

true if there may be other threads waiting to acquire
the lock.

hasContended

public final boolean hasContended()

Queries whether any threads have ever contended to acquire this
synchronizer; that is if an acquire method has ever blocked.

getQueueLength

public final int getQueueLength()

Returns an estimate of the number of threads waiting to
acquire. The value is only an estimate because the number of
threads may change dynamically while this method traverses
internal data structures. This method is designed for use in
monitoring system state, not for synchronization
control.

Returns:

the estimated number of threads waiting for this lock

getQueuedThreads

Returns a collection containing threads that may be waiting to
acquire. Because the actual set of threads may change
dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order. This method is
designed to facilitate construction of subclasses that provide
more extensive monitoring facilities.

Returns:

the collection of threads

getExclusiveQueuedThreads

Returns a collection containing threads that may be waiting to
acquire in exclusive mode. This has the same properties
as getQueuedThreads() except that it only returns
those threads waiting due to an exclusive acquire.

Returns:

the collection of threads

getSharedQueuedThreads

Returns a collection containing threads that may be waiting to
acquire in shared mode. This has the same properties
as getQueuedThreads() except that it only returns
those threads waiting due to a shared acquire.

Returns:

the collection of threads

toString

Returns a string identifying this synchronizer, as well as its
state. The state, in brackets, includes the String "State
=" followed by the current value of getState(), and
either "nonempty" or "empty" depending on
whether the queue is empty.

hasWaiters

Queries whether any threads are waiting on the given condition
associated with this synchronizer. Note that because timeouts
and interrupts may occur at any time, a true return
does not guarantee that a future signal will awaken
any threads. This method is designed primarily for use in
monitoring of the system state.

getWaitQueueLength

Returns an estimate of the number of threads waiting on the
given condition associated with this synchronizer. Note that
because timeouts and interrupts may occur at any time, the
estimate serves only as an upper bound on the actual number of
waiters. This method is designed for use in monitoring of the
system state, not for synchronization control.

getWaitingThreads

Returns a collection containing those threads that may be
waiting on the given condition associated with this
synchronizer. Because the actual set of threads may change
dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order.